Modern offset printing presses require the use of cleaning solvents. Traditionally solvents have been of the high volatility, petroleum based type, having relatively low boiling points. The solvents are used to clean the printing `blankets`, printing rollers, the press frames, and areas adjacent to the presses, and are typically referred to as "blanket and roller wash". The offset press printing blankets are rubberized sheets used to transfer the printing image to a moving paper, typically a web of newsprint, and as such must be cleaned regularly to remove the debris that builds up on the blanket during the printing process such as pigments, paper fibre lint and ink oils. Similarly the printing rollers which transfer the ink from the ink trays to the printing plates as well as the adjacent areas of the printing plant require cleaning. It is advantageous to be able to recover as much of the spent cleaning solvent as possible. First, recovery reduces the amount and disposal costs of waste chemical fluids released from the printing plant. Second, there is a savings to be made in re-using the solvent rather than purchasing virgin material.
In the blanket cleaning process, the solvents are normally mixed with water as water softens and loosens the paper fibres, thus allowing the solvents to penetrate and remove the inks. Typical recovery processes involve separating the solvent from water based diluents and from solids in the dirty mix, such as fibres, ink oils and pigments, by filtering, distillation and centrifuge methods. Filtering processes suitable for solvent recovery in the fine paper printing process may tend to be much less efficient with the dirty fluid mix from newspaper printing presses due to the much larger quantity of paper fibre. This paper fibre, or lint, combined with clays used in printing, tends to clog filters. The filter elements may be expensive, and the system may require a significant energy input as the pressure drop across the filter elements rises.
Distillation methods tend to consume more energy than may be desirable, and may not achieve satisfactory results when the fractions being separated by distillation have similar boiling points, or have a confused series of boiling points for various components. Another issue which may arise with distillation equipment is the need for improved fire protection if the distillation process includes flammable liquids or gases, and may require improved and expensive ventilation equipment to maintain a safe working environment. Conversely, the use of lower volatility solvents with higher boiling point ranges which produce reduced levels of toxic vapours in the work place air may tend to exacerbate the problem of distillation techniques because of the much higher temperatures required.
Centrifugal methods require a one or two stage centrifuge, typically an expensive piece of equipment, and typically have a relatively low overall processing rate. Ultrasonic filtering is also known, but once again requires more electrical equipment.
A second concern relates to the cleaning rags used to scrub the offset press blankets and to wipe the presses generally. At present these rags may be sent to a general laundry service which neither collects nor recycles the solvent, but rather may flush it away in the laundry wastewater. Cloth life between washing, solvent recovery, and cost savings could all be achieved if the solvent could be extracted from the cleaning rags and recycled.
It would be advantageous to have a relatively simple, low cost apparatus for reclaiming offset press washing solvent.